The infinite series description of trig functions is much neater when the argument is radians. For example, for small angles, sin(x) ≈ x when x is in radians. You could say that radians is the "natural" measurement unit for angles, just as "e" is the "natural" base of logarithms.
If the angle measure were degrees or grads or arcseconds, obnoxious scale factors would show up everywhere.
